1. Field of the Invention
The present invention relates to a high-power semiconductor laser with a high generation efficiency of light and a high fundamental-lateral-mode stability and a method for producing the same.
2. Description of the Related
Semiconductor lasers which oscillate laser beams in the visible region of the spectrum (visible laser diodes) have applications to such devices as laser beam printers and light sources for optical information processing devices such as optical disks, and therefore tend to have increased importance in these days. Materials of an (Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.40 P type, in particular, are attracting much attention because they can easily be lattice-matched to GaAs, which is an excellent material for substrates, and can oscillate laser beams having a wavelength in the range of 0.56 .mu.m to 0.68 .mu.m by varying the mole fraction x.
Hereinafter, a conventional semiconductor laser of a lateral-mode-control type having a doublehetero structure, which oscillates light of wavelengths pertaining to red regions, will be described.
As is shown in FIG. 10, this semiconductor laser includes an n-GaAs substrate 1, an n-GaAs buffer layer 2, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 3, a Ga.sub.0.51 In.sub.0.49 P active layer 4, a p-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 5, a p-Ga.sub.0.51 In.sub.0.49 P layer 6, an n-GaAs current blocking layer 13, and a p-GaAs capping layer 9 formed in this order. A p-side electrode 10 and an n-side electrode 11 are formed, respectively, upon the p-GaAs capping layer 9 and upon the bottom side of the substrate 1.
Fabrication of the semiconductor laser employs a crystal growing method such as a metal organic vapor phase epitaxy (MOVPE) method. By the use of such a crystal growing method, the n-GaAs buffer layer 2, the n-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 3, the Ga.sub.0.51 In.sub.0.49 P active layer 4, the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 5, and the p-Ga.sub.0.51 In.sub.0.49 P layer 6 are grown upon the n-GaAs substrate 1 in the respective order. Next, by photolithography and etching, the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 5 and the p-Ga.sub.0.51 In.sub.0.49 P layer 6 are etched so as to form a stripe-shaped ridge. Then, by an MOVPE method or the like, the n-GaAs current blocking layer 13 is selectively grown on the side slopes of the stripe-shaped ridge, and the p-GaAs capping layer 9 is grown on the n-GaAs current blocking layer 13. The p-side electrode 10 and the n-side electrode 11 are formed, respectively, upon the p-GaAs capping layer 9 and upon the bottom side of the substrate 1.
According to a semiconductor laser of this structure, a current can be confined by the n-GaAs current blocking layer 13. Moreover, by etching the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 5 to form stripe-shaped ridge having a trapezoidal cross section, effective refractive indices inside and outside stripe-shaped ridge can be made to have a difference which meets the requirements of a single lateral mode by optimizing the height and width of the trapezoid. As a result, light can be effectively contained in a predetermined region of the Ga.sub.0.51 In.sub.0.49 P active layer 4, the predetermined region being under the stripe-shaped ridge of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding layer 5. A typical width of each stripe of the stripe-shaped ridge is about 5 .mu.m.
However, in the above-mentioned conventional semiconductor laser shown in FIG. 10, the current blocking layer 13 is made of GaAs, which has a smaller energy band gap than that of the Ga.sub.0.51 In.sub.0.49 P active layer 4. As a result, the current blocking layer 13 serves to absorb light, that is, a large amount of light is lost so that a propagation loss .alpha..sub.i which occurs when light is propagated through a cavity of the semiconductor laser is as large as about 15 cm.sup.-1. This causes the external differential quantum efficiency to be as small as about 60%. Moreover, the kink level is as small as about 12 mW. Kinks are known to occur when the lateral mode shifts. In the semiconductor laser of the above structure, since the fundamental mode and higher modes have only small differences in gain, an increase of an injected current can easily cause one mode to shift to another; therefore kinks are likely to occur.
As has been described, the conventional structure cannot realize a semiconductor laser with a high output of light, due to the small external differential quantum efficiency and low stability of the fundamental lateral mode.